Construction of nuclear power plants is now positively promoted to meet the rapidly increasing demand for electric power. Most of the nuclear reactors in the nuclear power plants in operation at present are light-water reactors using as fuels uranium-235 which is contained in natural uranium in an among of only 0.7 wt.%. The amount of natural uranium deposits is estimated to be only about five million tons in the whole world. There is therefore a strong demand for the full industrialization of a nuclear power plant based on a fast breeder reactor which permits effective use of natural uranium of which the amount of deposits is limited as mentioned above.
A fast breeder reactor has the following advantages: The fast breeder reactor uses as fuels plutonium-239 and uranium-238 contained in large quantities in natural uranium. Nuclear fission of plutonium-239 is caused by fast neutrons, and this nuclear fission produces thermal energy. A fraction of fast neutrons produced through nuclear fission is absorbed into uranium-238 and converts uranium-238 into plutonium-239. As a result, converted plutonium-239 in an amount of over that of plutonium-239 consumed through nuclear fission is produced in the fast breeder reactor. With the fast breeder reactor, therefore, it is possible to produce thermal energy through nuclear fission of plutonium-239 over a long period of time without replenishing the fuels.
However, a nuclear power plant based on the fast breeder reactor requires a construction cost more than twice as high as that for a nuclear power plant based on the light-water reactor. Therefore, in order to achieve the full industrialization of the nuclear power plant based on the fast breeder reactor, reduction of the construction cost is essential.
The nuclear power plant based on the fast breeder reactor comprises a fast breeder reactor, a steam generator and an electric power generator. Thermal energy produced through nuclear fission of plutonium-239 as described above in the fast breeder reactor, heats liquid sodium as a coolant flowing through the fast breeder reactor to a high temperature. The thus heated high-temperature liquid sodium is introduced into the steam generator comprising a superheater and an evaporator, and heats high-pressure water flowing through the superheater and the evaporator through heat exchange. As a result, the high-pressure water flowing through the superheater and the evaporator becomes superheated steam. The thus produced superheated steam is fed to a turbine of the electric power generator to drive the turbine. Driving of the turbine causes electric power generation.
The superheater comprises a vessel, and heat exchanger tubes and tube sheets provided in the vessel. The temperature cf the superheater is increased to about 550.degree. C. by the superheated steam flowing through the heat exchanger tubes. Therefore, it is the conventional practice to use SUS304 austenitic stainless steel specified in JIS (Japanese Industrial Standards) as the material for the vessel of the superheater and to use SUS321 austenitic stainless steel specified in JIS as the material for the heat exchanger tubes and the tube sheets of the superheater.
The evaporator also comprises a vessel, and heat exchanger tubes and tube sheets provided in the vessel. The temperature of the evaporator is lower than that of the superheater. It is therefore the conventional practice to use 21/4Cr-1Mo steel as the material for the vessel, the heat exchanger tubes and the tube sheets of the evaporator.
The conventional use of expensive austenitic stainless steel as the material for the superheater causes the high construction cost of a nuclear power plant. Furthermore, the material for the superheater is different from that for the evaporator as described above. When connecting the superheater together with the evaporator by welding, therefore, the following problem is caused in the resulting welded joint: The carbon content of austenitic stainless steel which is the material for the superheater is lower than the carbon content of 21/4Cr-1Mo steel which is the material for the evaporator. The carbon activity of austenitic stainless steel in liquid sodium flowing through the superheater and the evaporator is different from that of 21/4Cr-1Mo steel. Consequently, decarburization occurs on the 21/4Cr-1Mo steel side in the welded joint during service and cementation, i.e., carburization takes place on the austenitic stainless steel side in the welded joint, thus resulting in deterioration of the welded joint.
With a view to solving the above-mentioned problems, a low-cost heat-resistant steel having a creep strength comparable with that of the above-mentioned austenitic stainless steel is required as the material common to the superheater and the evaporator. As a heat-resistant steel meeting such a requirement, ASTM (American Society for Testing and Materials) Standards specify a 9% chromium heat-resistant steel (A213-T91) having the chemical composition as shown in Table 1.
TABLE 1 ______________________________________ C Si Mn P S Cr Mo V Nb ______________________________________ 0.10 0.39 0.38 0.002 0.006 8.30 0.93 0.21 0.08 ______________________________________
However, the 9% chromium heat-resistant steel (A213-T91) having the chemical composition shown in Table 1 has the following problems: The carbon content is so high as 0.10 wt.%. Low-temperature cracking resistance in the welded joint is therefore low, and the production of .alpha.+.gamma. phase upon solidification of molten metal during welding results in a low high-temperature cracking resistance in the welded joint. In addition, since creep strength of the base metal becomes excessively high, there occurs a large difference in creep strength between the softened zone of the welded joint and the base metal, thus resulting in deterioration of the welded joint.
As a low-cost heat-resistant steel having a creep strength comparable with that of the above-mentioned austenitic stainless steel, JIS specifies a 9% chromium heat-resistant steel (STBA-27) having the chemical composition shown in Table 2 (although not as yet officially instituted).
TABLE 2 ______________________________________ C Si Mn P S Cr Mo ______________________________________ 0.05 0.46 0.55 0.002 0.007 8.47 2.00 ______________________________________
However, the 9% chromium heat-resistant steel (STBA-27) having the chemical composition shown in Table 2 has the following problems: The molybdenum content is so high as 2.00 wt.%. This causes an increase in the amount of ferrite in the steel, thus resulting in low toughness. In addition, when heated for a long period of time during service, precipitation of a Laves phase (Fe.sub.2 Mo) leads to a further deterioration of toughness.
The nuclear power plant based on the fast breeder reactor requires a high construction cost as described above. Therefore, in order to cover the huge construction cost and to reduce the electric power generation cost to below that of an electric power plant using coal, petroleum or liquefied natural gas as the fuel, it is necessary to increase the operating rate of the plant without the occurrence of accidents.
Under such circumstances, there is a strong demand for the development of a low-cost chromium heat-resistant steel which is excellent in toughness and has a high cracking resistance and a high creep strength when said steel is utilized to form a welded joint, and which is particularly suitable for use as the material for a steam generator of a nuclear power plant based on a fast breeder reactor, but such a heat-resistant steel has not as yet been proposed.